1. Technical Field
The present invention generally relates to pneumatic tire construction and, more particularly, to the structure of a runflat tire. Specifically, the present invention relates to a cantilever runflat tire having a sidewall insert and a stiffener member that work together to provide desirable ride and handling characteristics to the tire.
2. Background Information
A runflat tire is known in the art as a pneumatic tire that safely operates in a deflated (zero PSI) condition. Typical runflat tires include a pair of radial crescent shaped reinforcing sidewall elements fabricated from at least one solid band of high modulus low hysteresis material built into the sidewall of a conventional tire. Runflat tires are becoming more desired in today""s vehicles because a vehicle using four runflat tires does not have to carry a spare tire. Spare tires are undesirable because they add weight to the vehicle and consume valuable storage space in a vehicle. Runflat tires are also desirable because they make vehicles safer by preventing a driver from having to change a tire in an unsafe location.
Storing spare tires has become increasingly problematic in today""s sport utility vehicles because the vehicles are using larger tires with larger tire rims. One solution to the size problem has been to provide a high inflation pressure mini spare tire that may be used by the vehicle owner as a temporary solution until the vehicle may be driven to a service station. As consumer""s tastes have changed toward larger vehicles with larger rims, the mini spare tire has grown to such a large size that the purpose of providing the mini spare has been defeated. The art thus desires runflat tires that may be used on sport utility vehicles and trucks. These runflat tires must provide acceptable inflated ride characteristics while being used in situations where a high aspect ratio tire is called for. The art particularly desires runflat tires for situations where conventional tires having aspect ratios of 65 and higher are required.
The design of prior art runflat tires has been limited based on durability requirements and ride characteristics. The art recognizes that uninflated durability varies inversely as aspect ratio increases in prior art runflat tires. For example, a typical prior art tire having an aspect ratio of 45 may run hundreds of uninflated kilometers while a typical prior art tire having an aspect ratio of 60 may only operate for 80 uninflated kilometers. The unofficial industry standard for runflat tires is at least 50 miles at 55 miles per hour (80 kilometers at 88 kilometers per hour). A successful runflat tire to be used in high aspect ratio applications must meet or exceed this standard while providing acceptable ride characteristics.
The art generally recognizes that a lower aspect ratio tire has better handling characteristics than a higher aspect ratio tire while having decreased ride characteristics. Similarly, runflat tires having crescent-shaped sidewall inserts generally have improved handling characteristics while having decreased ride characteristics.
Prior art cantilever tires such as the tire disclosed in U.S. Pat. No. 3,486,547 provide a better combination of ride and handling characteristics. For instance, a cantilever tire can provide increased handling characteristics at a similar ride quality as a conventional tire. The art generally recognizes that cantilever tires include a pair of cantilevered sidewall portions that extend generally parallel to the ground adjacent the tire rim compared to a conventional tire. The cantilevered portion acts as a stabilizer ring that allows flexibility in the radial direction and provides stiffness in other directions. The cantilevered portion of the sidewall adjacent the rim resists lateral distortion during cornering. For lateral loads, a cantilever tire responds as if it had an effective rim width equal to the width of the rim plus the width of the cantilevered portions. The cantilever tire achieves the improved handling characteristic while not adversely affecting the ride characteristics of the tire.
In view of the foregoing, the present invention provides a runflat tire having the improved handling characteristics of a conventional tire while having the improved ride characteristics of a cantilever tire. The invention provides a cantilever runflat tire having a sidewall insert that provides the tire with sufficient radial stiffness in an uninflated condition to have acceptable uninflated durability. The cantilever runflat tire also includes a cantilever sidewall portion that provides desirable inflated ride characteristics. The cantilever runflat may be used in situations previously requiring a conventional high aspect ratio tire.
The cantilever runflat tire of the present invention includes a pair of axially-spaced bead portions with each having a bead filler and a bead core. The tire includes a pair of axially-spaced sidewalls with each of the sidewalls including an insert. Each of the sidewalls has a radial portion and a cantilever portion cantilevered with respect to the bead core.
The cantilever runflat tire has the advantage that the sidewall insert may be relatively thin and relatively lightweight compared to the amount of uninflated durability it provides to the tire. The thin sidewall insert also does not drastically increase the vertical spring rate of the runflat cantilever tire compared to a conventional cantilever tire. The cantilever runflat tire thus has desirable inflated ride characteristics while maintaining desirable uninflated durability.
The cantilever runflat tire of the invention also provides desirable ride and handling characteristics by including a stiffener member and sidewall insert fabricated from a hard, high modulus rubber compound having a Shore A Durometer between 70 and 97, a mechanical static modulus in the range of 1400 psi (9651 KPa) to 4000 psi (27576 KPa) at 15% strain, and loss tangent delta (tan xcex4) between 0.03 to 0.2 measured at 100xc2x0 C., 7% deflection, and 10 Hz. The stiffener member may also include reinforcing members such as fabric or cords.